All my user accounts have disappeared

Hi i need help please.
I am trying to rescue my neighbours pc but am running out of ideas on how to solve it.
This is the problem:-
The pc has 5 user accounts in XP Home editon. But when you get to the welcome screen, the right hand side, where the names and pictures of the users are, is blank. only one account is acessable by pressing Ctrl,alt,delete twice.
When you eventually get to the desktop, everything appears to work fine except when you go into Control Panel and click on User Accounts you receive the error message "The current user account is not recognised. Restart the computer and then go into user accounts".
I have tried this also in safe mode with the same result. I have cleared the system off all spyware etc (to the best of my knowlegde), and tried a repair with the installtion disk.
I just wondered if anyone had a solution before i just do a clean install.
Only Service Pack 1 is installed.